It is well known to provide computing devices such as personal computers (PCs), servers, media centers and the like with power supply units (PSUs). A PSU receives mains electricity, and provides regulated direct current (DC) power at one or more outputs. Since different components of computing devices have different power supply requirements, it is usual to provide at different outputs power at different voltages and with different maximum power ratings.
A PSU typically comprises a cuboid housing made of a metallic sheet material. The metal prevents electromagnetic interference outside of the housing, which interference may negatively affect the operation of other components in the computing device. The housing houses components including one or more transformers and one or more rectifiers which convert mains power, received via a mains power connector mounted in one side wall of the housing, into the required power supplies.
The side wall of the housing with the mains power connector is often also provided with an on/off switch. This wall is then supported in or against an aperture of a casing of the computing device when installed, so as to allow a mains power cable to be connected to the PSU. This makes it convenient for the wall also to be used to allow air to be expelled from the interior of the PSU housing.
The total power supplied by a PSU typically is of the order of hundreds of Watts. The components required to generate such power necessarily generate waste heat within the housing which needs to be removed. Three different types of arrangement can be used to achieve this. Each uses one or two fans.
A fan includes a motor and a fan shroud comprising blades mounted on a support and rotatable by the motor. The fan shroud in generally circular, and rotation of it by the motor causes air to be forced in a direction perpendicular to the plane of the fan component. A fan is supported next to an aperture in a wall of the housing. The aperture is about the same size and shape as the fan component. To prevent a user's fingers or similar being contactable with the blades of the fan, a grill is normally secured to the housing so as to cover the aperture without restricting significantly the flow of air through the fan aperture. This grill also provides grounding to prevent electromagnetic interference (EMI).
In one type of arrangement, two fans are provided, a first fan being supported with the upper wall and a second fan being supported with the side wall in which the mains connector is provided. In this type of arrangement, the first fan blows inwards of the PSU and the second fan blows outwards. As a result, air flows into the PSU through the upper wall, is heated by the internal components of the PSU and the heated air then is expelled by the second fan to the exterior of the housing of the computing device. To minimise the influx of air heated by the components of the PSU into the housing of the computing device containing the PSU, the PSU housing in such an arrangement is not provided with apertures, although a small amount of heated air will sometimes be able to escape the PSU around connectors, wire harness exit apertures and the like. Heat also is conducted through the walls of the housing.
In another type of arrangement, a PSU includes a fan in the side wall that includes the mains power connector, and no other fans. The fan is arranged to blow outwards of the PSU. To allow air to enter the PSU, the side wall opposite the side wall including the mains power connector is provided with apertures. Thus, the fan produces a reduction in air pressure within the PSU housing. Air then flows through the apertures by virtue of higher pressure air in the computing device housing, is heated by the PSU components and is expelled to atmosphere by the fan. To produce maximum cooling for a given fan throughput, the other side walls and the upper wall of the PSU housing are not usually provided with apertures. This maximises the amount of air expelled by the fan being air which has passed over a heat-generating component of the PSU. The SilentX 300W PSU produced by Ahanix is one example of this type of arrangement. This PSU includes a series of parallel elongate slits some tens of mm long and a few mm wide in the side wall opposite the side wall which includes the mains power connector.
In a third type of arrangement, a single fan is included on an upper wall of the PSU and is arranged to blow inwards of the PSU. In this case, an exit for air is provided on the side wall which includes the mains power connector. This exit can take any of a number of different forms, for example a grill. The other side walls are not provided with apertures, which enhances the channelling of air over the heat-generating components of the PSU. One example is the FSP300-60GLS produced by the FSP group of Taiwan. This PSU has a mesh on the side wall on which the mains connector is formed. The mesh allows air forced into the PSU by the fan to be expelled to the exterior of the housing of the computing device in which it is included.
In a fourth type of arrangement, an inwardly blowing fan is provided on the rear faces, and an outwardly blowing fan is provided on the front face. This makes the housing a tunnel through which air is blown.
In a fifth type of arrangement, inwardly blowing fans are provided on both upper and rear faces, and an outwardly blowing fan is provided on the front face. This arrangement tends to be effective, but requires three fans so is more expensive.
It is known also to use two fans mounted adjacent one another in one wall of the PSU housing. These fans then operate in parallel, and can be considered to be equivalent to a single fan of greater capacity mounted in that wall of the PSU housing.
It is known in some multiple fan PSUs to include a small number of elongate slits over an area of a side wall. Such slits serve to compensate for any differences in the rate of air entering and leaving the housing by way of the fans. Thus, if inwardly blowing fans have a greater flow rate than outwardly blowing fans, then the slits allow air to leave the housing of the PSU. Without the slits, the inwardly blowing fans would be more stressed. Similarly, if outwardly blowing fans have a greater flow rate than inwardly blowing fans, then the slits allow air to enter the housing of the PSU. Without the slits, the outwardly blowing fans would be more stressed.
Fanless PSUs are known, but suffer from certain disadvantages.
Although in a conventional fan PSU one or more fans are used to circulate air through the PSU, the presence of internal components means that it is not always possible to ensure that there is sufficient air movement in all of the volume formed by the housing. As such, it is possible for air in some locations to be heated to a temperature significantly above the average air temperature within the housing. Such heated air is termed a hotspot. Since hot air is less dense than cooler air, hotspots move once formed. When a hotspot moves to a location where there is a temperature sensitive material or component, such as the plastic insulation of a wire, the plastic of a connector or a packaged semiconductor device, that material or component can become damaged. Since hotspots usually rise within the PSU, damage is most likely to occur to components located near the upper wall of the housing.
The formation of hotspots can cause a fan PSU to cease working correctly, and in some cases cease functioning altogether. For this reason, fan PSU designers choose fans having flow rates sufficiently high to reduce the probability of hotspots forming and causing damage to a minimum. This normally means that the flow rates of the fans are higher than would be the case if hotspots could not cause problems. However, higher flow rate fans are generally more expensive and/or noisier than lower flow rate fans. Thus, the need to avoid hotspots increases to the cost of fan PSUs and/or the noise levels that are generated by them.